Molecular Characterization of Mismatch Repair Deficient Tumors In Young Jordanian Patients

In Jordan, approximately 19% of colorectal carcinomas (CRCs) in patients younger than 45 years show mismatch repair deficiency (MMRD) by immunohistochemistry (IHC). MMRD in pediatric high-grade gliomas (HGG) and CRC in Jordan appears substantially more frequent than global estimates (39% and 44%, respectively), likely reflecting high consanguinity rates and an increased contribution of constitutional MMRD (CMMRD). However, molecular characterization of MMRD-associated tumors in the Middle East remains limited. We analyzed 14 Jordanian patients (< 45 years) from 12 families with clinical or IHC evidence of MMRD, including four brain tumors and 11 CRCs; five patients were children. Pathogenic or likely pathogenic MMR variants were identified in nine families, including one affected sibling pair, comprising five frameshift variants, three single nucleotide variants, and one large deletion. A variant of uncertain significance in MSH6 was detected in another sibling pair. Apparent large deletions suggestive of sequencing artifacts were observed in six patients, underscoring the need for confirmatory copy number analysis by multiplex ligation-dependent probe amplification (MLPA). Three adult cases lacked germline MMR variants, MLH1 hypermethylation, or alterations in other cancer predisposition genes and were considered likely somatic. Tumor mutational burden (TMB) ranged from 2 to 352 variants/Mb (median: 23). Microsatellite instability (MSI) was low in seven tumors, likely due to degraded DNA from older Formalin-Fixed Paraffin-Embedded (FFPE) samples. Among tumors with low TMB and MSI, only one lacked an identifiable germline MMR variant. All tumors showed overrepresentation of C > T transitions, consistent with MMRD-associated mutational signature 6. This study represents the first comprehensive molecular characterization of MMRD-associated tumors in Jordan.